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EQ3/6, a program for calculation of geochemical equilibrium in aqueous systems and which can be downloaded here, has a spectacularly unhelpful Install.txt file.

Sadly, we do not provide specific instructions on how to compile and link
the software to build the executables. You have to figure that out yourself
for your specific platform. It will require some number of steps. Also, we
do not provide makefiles. It is not necessary to use make files, although
some people like them. If all this seems to exceed your knowledge and capacity,
seek the assistance of a qualified Linux or Unix maven who knows how to do
these things on your platform.

2. Extract it. Move the EQ3_6v8.0a folder to ~
You can put it wherever you want, but then need to edit eq36cfg, and you'll have to deal with csh not handling long paths that well. It's just easier to give up and put EQ3_6v8.0a under $HOME

eq6.f:16.9:
use mod6pt
1
Fatal Error: Can't open module file 'mod6pt.mod' for reading at (1): No such file or directory
eqcalc.f:77.9:
use mod6pt
1
Fatal Error: Can't open module file 'mod6pt.mod' for reading at (1): No such file or directory
eqphas.f:129.9:
use mod6pt
1
Fatal Error: Can't open module file 'mod6pt.mod' for reading at (1): No such file or directory
eqshel.f:146.9:
use mod6pt
1
Fatal Error: Can't open module file 'mod6pt.mod' for reading at (1): No such file or directory
exivar.f:121.9:
use mod6pt
1
Fatal Error: Can't open module file 'mod6pt.mod' for reading at (1): No such file or directory

[..]
Done. Optimization ended outside requested limits.
Starting hybrid Newton-Raphson iteration.
Done. Hybrid Newton-Raphson iteration converged in 56 iterations.
* Warning - (EQ3NR/eq3nr) The calculated TDS of 3.44395E+05 mg/L
differs from the input file value of 3.54016E+05 mg/L
by more than 1%. The calculated value will be used in
subsequent calculations.
The pickup file has been written.
No further input found.
Start time = 13:57:15 21Mar2017
End time = 13:57:15 21Mar2017
Run time = 0.586E-01 seconds
Normal exit
Note: The following floating-point exceptions are signalling: IEEE_UNDERFLOW_FLAG
------------------------------------
The following output files were written:
c4pgwbN2.3o
c4pgwbN2.3p
------------------------------------
The following input files were run without generating
any EQ3NR error messages:
c4pgwbN2.3i
------------------------------------
All done

The question here is whether the error is indicative of a serious issue with the compiled software.

I reran as

csh ../scripts/runeq3 fmt c4pgwbN2.3i |tee c4pgwbN2.3o

and compared

diff c4pgwbN2.3o Outputs/c4pgwbN2.3o

18c18
< Run 14:12:44 21Mar2017
---
> Run 13:18:06 07Sep2011
870,871c870,871
< -0.0000 per cent of the total charge
< -0.0000 per cent of the mean charge
---
> 0.0000 per cent of the total charge
> 0.0000 per cent of the mean charge
1235,1236c1235,1236
< Start time = 14:12:44 21Mar2017
< End time = 14:12:44 21Mar2017
---
> Start time = 13:18:06 07Sep2011
> End time = 13:18:06 07Sep2011
1238c1238
< Run time = 0.586E-01 seconds
---
> Run time = 0.781E-01 seconds

The results are the same.We're good.
Everything put together as a script:

When I hit 0 to visualize something, the plot screen is empty. Check/uncheck a box in the window and the plot will show.
Generating wfx files in gaussian:
If you've already run a job that has generated a .chk file, then run a simple quick job like this:

To analyse the .wfx file using multiwfn, look at the examples in the manual: https://www.codeplex.com/Download?ProjectName=multiwfn&DownloadId=1607667
The strength of multiwfn is that it can do a LOT. That's also its weakness, in the sense that the menu options are a bit overwhelming. Following the examples in the manual is probably mandatory.